Determination of effective composition of a mixture of hydrocarbon gases

ABSTRACT

A method and apparatus for determining the effective composition of a mixture of gases including a plurality of hydrocarbon gases, the method comprising selecting one or more effective hydrocarbons to represent the plurality of hydrocarbon gases in the gas mixture, the number of effective hydrocarbons being less than the number of hydrocarbon gases in the gas mixture whose composition is to be determined; measuring a number of characteristics of the gas mixture whose effective composition is to be determined, the number of characteristics to be measured being one less than the total number of components to be determined and determining the effective composition of the mixture of gases from the measurements of the characteristics of the gas mixture, a predetermined parameter dependent upon the characteristic measured and knowing that the sum of the components of the gas mixture equals 100%.

[0001] The present invention relates to the determination of an effective composition of a mixture of hydrocarbon gases such as natural gas. The effective composition can be used to determine a quantity indicative of the quality of the gas such as its calorific value, relative density and Wobbe index.

[0002] Conventionally to determine the quality of a gas mixture such as its calorific value one could determine the proportion of each of the gases in the mixture and calculate the calorific value. However, for a mixture of several gases such as natural gas it is difficult to determine the proportion of each and every gas.

[0003] According to a first aspect of the present invention a method of determining an effective composition of a mixture of hydrocarbon gases comprises representing a mixture of hydrocarbon gases by an effective mixture of fewer hydrocarbon gases and determining the effective proportion of each of the hydrocarbon gases in the effective mixture.

[0004] The effective proportions of each of the hydrocarbon gases in the effective mixture is preferably used to determine a parameter indicative of the quality of the gas such as calorific value(CV), relative density (RD) or Wobbe index(WI).

[0005] An example of the invention will now be described with reference to the accompanying drawings in which:

[0006]FIG. 1 shows CV determined in accordance with the present invention compared to actual values;

[0007]FIG. 2 shows RD determined in accordance with the present invention compared to actual values; and

[0008]FIG. 3 shows an apparatus for performing the invention.

[0009] It has been found that a mixture of hydrocarbon gases such as natural gas can be represented by a reduced number of components. For example the many hydrocarbons in a natural gas can be represented by one or an effective mixture of two or more hydrocarbons. In the following example the hydrocarbons in a natural gas are represented by a mixture of effective methane (CH₄) and propane (C₃H₈). The proportions of the individual components of the effective gas are selected to produce the same major properties as the gas itself. The effective gas for the other hydrocarbons is the volume of propane and methane that has the same ideal volume and the same average number of carbon atoms per molecule as the gas under consideration. For example, consider ethane:

2C₂H₆═C₃H₈+CH₄

[0010] Ethane is represented in the effective gas by half its volume of propane and half its volume of methane. The effective factors for ethane in terms of propane and of methane are therefore, 0.5 and 0.5. Other hydrocarbons can be represented in these terms and their effective factors are tabulated below: Effective Hydrocarbon CH₄′ C₃H₈′ Methane 1.0 — Ethane 0.5 0.5 Propane — 1.0 Butane −0.5 1.5 Pentane −1.0 2.0 Hexane −1.5 2.5

[0011] Natural gas generally also contains CO₂ and N₂ and so, assuming that the hydrocarbons are represented by an effective mix of methane and propane, the natural gas can be represented by a four component mixture of methane, propane, CO₂ and N₂. If the hydrocarbons are represented by just one hydrocarbon then the natural gas could be represented by an effective three component mix of the effective hydrocarbon, CO₂ and N₂.

[0012] An example is given below of a natural gas represented as an effective four component mixture of methane, propane, CO₂ and N₂. The actual composition of a sample of natural gas identified as bottle 2912 is as follows: CH₄ C₂H₆ C₃H₈ N—C₄H₁₀ I—C₄H₁₀ N—C₅H₁₂ CO₂ N₂ CV Bottle ID % mol % mol % mol % mol % mol % mol % mol % mol MJ/m³ 2912 94.451 3.11 0.512 0.109 0.089 0.170 0.647 0.912 38.73

[0013] Applying effective methane and propane calculations to the hydrocarbon samples of the above natural gas sample gives the following result: Hydrocarbon CH₄′ C₃H₈′ Methane 1.0 94.451 94.451 — 94.451 0 Ethane 0.5 3.110 1.555 0.5 3.110 1.555 Propane — 0.512 0 1.0 0.512 0.512 Butane −0.5 0.198 −0.099 1.5 0.198 0.297 Pentane −1.0 0.170 −0.170 2.0 0.170 0.340 Hexane −1.5 0 0 2.5 0 0 CH₄′ = 95.737 C₃H₈′ = 2.704

[0014] Thus the hydrocarbons of the natural gas sample can be represented by an effective mixture of 95.737% methane and 2.704% propane. The CO₂ and N₂ samples can be included to provide the four component effective natural gas mixtures: CH₄′ C₃H₈′ CO₂ N₂ Σ Bottle ID % mol % mol % mol % mol % mol 2912 95.737 2.704 0.647 0.912 100

[0015] A Method has been found to determine the effective methane, propane, CO₂ and N₂ proportions in a sample of natural gas. It is known that the sum of the four components equals 100%. By making three measurements of the characteristics of the natural gas and knowing that the sum of the four components equals 100% the relative effective proportions of methane, propane, CO₂ and N₂ can be determined using simultaneous equations as shown below:

X=C1.CH₄+C2.C₃H₈+C3.CO₂+C4.N₂

Y═C5.CH₄+C6.C₃H₈+C7.CO₂+C8.N₂

Z=C9.CH₄+C10. C₃H₈+C11.CO₂+C12. N₂

100=CH₄%+C₃H₈%+CO₂%+N₂%

[0016] X, Y and Z are measurements taken of the sample of gas being investigated. In this case X is the thermal conductivity of the gas at a first temperature (THCst), Y is the thermal conductivity of the gas at a second temperature (ThCr) and Z is the speed of sound in the gas (SOS). The thermal conductivities of the gas at the two temperatures and the speed of sound of the gas are measured using any suitable technique as is well known in the art. Any convenient characteristics of the gas may be measured and used as X, Y or Z. The parameters C1 to C12 are constant for a particular measurement apparatus, and a particular temperature and pressure and the particular characteristic being measured. The values for C1 to C12 may be found by calibrating a particular apparatus using a gas of known effective methane, effective propane, CO₂ and N₂ proportions.

[0017] To determine the coefficients C1 to C12 at a fixed temperature and pressure, a set of data is determined which maps values of X, Y and Z for the expected range in gas composition. Regression analysis is then used with the gas components as the X input range and parameter X as the Y input range, the resulting analysis gives C1 to C4 at this temperature and pressure. This process is repeated with parameter Y as the Y input to give C5 to C8 and again with parameter Z as the Y input to give C9 to C12. This gives C1 and C12 at a fixed temperature and pressure.

[0018] These coefficients are temperature and pressure dependent. So the whole process needs to be repeated for a range of temperatures and pressures covering the specified operating range. Once this is complete a mathematical model can be determined to predict C1 to C12 at any temperature and pressure in the specified range.

[0019] The effective proportions of methane, propane, CO₂ and N₂ may be determined from the measured gas characteristics (in this example two thermal conductivities at different temperatures and the speed of sound) using a matrix method as shown below: ${\begin{pmatrix} {C1} & {C2} & {C3} & {C4} \\ {C5} & {C6} & {C7} & {C8} \\ {C9} & {C10} & {C11} & {C12} \\ 1 & 1 & 1 & 1 \end{pmatrix}^{- 1}\begin{pmatrix} {ThC}_{st} \\ {ThC}_{r} \\ {SOS} \\ 100 \end{pmatrix}} - \begin{pmatrix} {CH}_{4}^{\prime} \\ {C_{3}H_{8}^{\prime}} \\ {CO}_{2} \\ N_{2} \end{pmatrix}$

[0020] Once the effective proportions of methane, ethane, CO₂ and nitrogen and are known, these can be used to determine the calorific value (CV), relative density (RD) and Wobbe Index (WI) as explained below.

[0021] The ideal gas superior calorific value (water vapour condensed) on a volumetric basis, for a combustion temperature t₁, of a mixture of known composition metered at a temperature t₂ and pressure p₂, can be calculated using: ${{\overset{\sim}{H}}_{s}^{o}\left\lbrack {t_{1},{V\left( {t_{2},p_{2}} \right)}} \right\rbrack} = {\sum\limits_{J = 1}^{N}{x_{j} \cdot {{\overset{\sim}{H}}_{j}^{o}\left\lbrack {t_{1},{V\left( {t_{2},p_{2}} \right)}} \right\rbrack}}}$

[0022] Where:

[0023] {tilde over (H)}^(o) _(s) [t₁. V(t₂,p₂] is the ideal superior caloric value on a volumetric basis of the mixture.

[0024] Xj if the mole fraction of component j determined from the earlier effective constituent analysis.

[0025] {tilde over (H)}^(o) _(s)[t1, T(t2,p2)] is the ideal superior calorific value on a volumetric basis of component j.

[0026] In the UK, Metric Standard Conditions (MSC) are preferred:

[0027] P₂ is 101.325 KPa(1.01325bar A)

[0028] t₁=t₂=15° C. (288.15K)

[0029] Calorific Values for methane and propane at metric standard conditions are known: Component {tilde over (H)}^(o) _(j) [15, V (15, 101.325)} CH₄ 37.706 C₃H₈ 93.940

[0030] Multiplying these values by the proportions of methane and propane determined earlier provides a value for

{tilde over (H)}^(o) _(s)[t₁,V(t₂,p₂)]

[0031] The real-gas caloric value on a volumetric basis is calculated from the equation: ${{\overset{\sim}{H}}_{s}\left\lbrack {t_{1} \cdot {V\left( {t_{2},p_{2}} \right)}} \right\rbrack} = \frac{H_{s}^{o}\left\lbrack {t_{1},{V\left( {t_{2},p_{2}} \right)}} \right\rbrack}{Z_{mix}\left( {t_{2},p_{2}} \right)}$

[0032] Where:

[0033] {tilde over (H)}₅ (t₁.V(t₂, p₂)] is the real-gas superior calorific value on a volumetric basis.

[0034] The compression factor Z_(mix) at the metering conditions is calculated using: ${Z_{mix}\left( {t_{2},p_{2}} \right)} = {1 - \left\lbrack {\sum\limits_{J = 1}^{N}{X_{j} \cdot \sqrt{b_{j}} \cdot}} \right\rbrack^{2}}$

[0035] Where

[0036] {square root}b_(j) is the so-called summation factor which for methane is 0.0447 and for propane is 0.1338.

[0037]FIG. 1 shows the CV determined as above using the effective composition plotted on the y-axis and the actual CV along the x-axis.

[0038] The CV calculated for the effective composition is within +/−0.008 MJ/m³ (2 standard deviations) of the actual CV along the x-axis.

[0039] The relative density (RD) of a gas is calculated using the following: $d^{o} = {\sum\limits_{J = 1}^{N}{X_{j} \cdot \frac{M_{j}}{M_{air}}}}$

[0040] Where:

[0041] d^(o) is the relative density of the ideal gas

[0042] M_(j) is the molar mass of component j

[0043] M_(air) is the molar mass of dry air of standard composition (28.9626 kg.mol⁻¹)

[0044] The relative density of the real gas is calculated from: ${d\left( {t,p} \right)} = \frac{d^{o} \cdot {Z_{air}\left( {t,p} \right)}}{Z_{mix}\left( {t,p} \right)}$

[0045] Where:

[0046] d(t,p) is the relative density of the real gas

[0047] Z_(air) (t,p) is the compression factor of dry air of standard composition (MSC=0.99958)

[0048] The compression factor Z_(mix) at the metering conditions is calculated using; ${Z_{mix}\left( {t,p} \right)} = {1 - \left\lbrack {\sum\limits_{J = 1}^{N}{X_{j} \cdot \left\lbrack b_{j} \right\rbrack^{2}}} \right.}$

[0049] Where:

[0050] {square root}b_(j) is the so-called summation factor which for methane is

[0051] 0.0447 and for propane is 0.1338.

[0052]FIG. 2 shows the relative density determined assuming an effective composition of methane, propane, CO₂ and N₂ for various gas samples plotted on the y-axis and the actual relative density of those gas samples plotted on the x-axis.

[0053] The relative density calculated for the effective composition is within +/−O0.00001 (2 standard deviations) of the actual values.

[0054] The Wobbe Index (WI) of a gas may be determined from the CV and RD calculated above using the equation: ${WI} = \frac{CV}{\sqrt{RD}}$

[0055] The effective composition concept can be used with other standard methods to calculate such properties as compressibility, density, etc.

[0056] The invention may be performed by an apparatus using an appropriate number of sensors or detectors, in this case a thermal conductivity detector arranged to measure the thermal conductivity of a gas at two temperatures and a speed of sound detector; and a control means to perform the appropriate processing which could be a computer for example. Such an apparatus is shown in FIG. 3. A fluid in this case, natural gas, is supplied in a conduit 1. A thermal conductivity sensor 2 having means to measure the thermal conductivity of the natural gas at two different temperatures is provided in the conduit. A device 3 to measure the speed of sound of the natural gas is also provided. The thermal conductivity sensor 2 and the speed of sound sensor 3 are both connected to a control means 4, in this case a computer to receive signals indicative of the thermal conductivity of the passing natural gas at two different temperatures and the speed of sound of the passing natural gas to determine the effective composition of the passing natural gas. The control means 4 may also use the determined effective composition to calculate a quantity indicative of the quality of the gas.

[0057] The method of the invention may be provided on a digital storage media such as a CD-ROM for installation on a suitable apparatus such as a computer connected to appropriate sensors on detectors. 

1. A method for determining the effective composition of a mixture of gases including a plurality of hydrocarbon gases, the method comprising selecting one or more effective hydrocarbons to represent the plurality of hydrocarbon gases in the gas mixture, the number of effective hydrocarbons being less than the number of hydrocarbon gases in the gas mixture whose composition is to be determined; measuring a number of characteristics of the gas mixture whose effective composition is to be determined, the number of characteristics to be measured being one less than the total number of components to be determined and determining the effective composition of the mixture of gases from the measurements of the characteristics of the gas mixture, a predetermined parameter dependent upon the characteristic measured and knowing that the sum of the components of the gas mixture equals 100%.
 2. A method according to claim 1, wherein the mixture of gases whose composition is to be determined is natural gas which includes carbon dioxide and nitrogen in addition to a plurality of hydrocarbon gases.
 3. A method according to claim 2, wherein three gas characteristics are measured such that a total of four components are determined for the natural gas, two components being effective hydrocarbons, one being carbon dioxide and one being nitrogen.
 4. A method according to claim 3, wherein the two effective hydrocarbons are methane and propane.
 5. A method according to claim 3 or claim 4, wherein the three gas characteristics that are measured are thermal conductivity at a first temperature, thermal conductivity at a second temperature and the speed of sound in the natural gas.
 6. Determination of the calorific value, relative density or Wobbe index of natural gas from an effective composition of the gas determined according to any of the preceding claims.
 7. An apparatus for determining the effective composition of a mixture of gases including a plurality of hydrocarbon gases, the apparatus comprising control means for selecting one or more effective hydrocarbons components to represent the plurality of hydrocarbon gases in the gas mixture, the number of effective hydrocarbons being less than the number of hydrocarbon gases in the gas mixture whose composition is to be determined; measuring means for measuring a number of characteristics of the gas mixture whose effective composition is to be determined, the number of characteristics to be measured by the measuring means being one less than the total number of components to be determined and determining the effective composition of the mixture of gases from the measurements of the characteristics of the gas mixture, a predetermined parameter dependent upon the characteristic measured and knowing that the sum of the components of the gas mixture equals 100%. 